1. Field of the Invention
The present invention relates to techniques for deriving statistical device models for mass-produced products such as semiconductor chips.
2. Description of the Related Art
During the mass manufacture of various products, such as semiconductor chips or systems having integrated circuits (ICs), failures can occur in some of the chips produced, i.e., some of the chips are defective. For example, in mass-producing semiconductor chips, chip-to-chip variations, sometimes referred to as interdie variations, can be introduced during the fabrication process. If these variations are too far from the ideal or nominal for a given chip, the chip may not function properly because various key circuit-level performance metrics may vary too much from the maximum tolerable limits.
The individual units of the product which is mass produced may be sometimes referred to herein as chips. Chips are formed in the substrate of a physical wafer. Typically, several chips are formed in each wafer. A set of wafers that are processed together is called a lot. A wafer is a very thin, flat disc typically about 9xe2x80x3 in diameter. The manufacturing process consists of operations on the surface and substrate of the wafer to create a number of chips. Once the wafer is completely processed, it is cut up into the individual chips, which are typically about half inch by half inch in size. A lot is thus a mass-produced set of chips or units, each of which is supposed to conform to an ideal design within certain tolerable limits.
Inevitable variations in the manufacturing process can give rise to interdie variations, which can be more or less severe depending upon the particular manufacturing process as well as upon the particular design of the product to be mass-produced. The number of chips that satisfy all performance specifications from a lot determine the parametric yield of the design and manufacturing process used with a given target foundry for mass producing the chip. Each chip 30 comprises an IC or system which itself comprises a network of several circuit-level elements such as operational amplifiers (op amps) and the like. All of these circuit-level elements are composed of so-called compact or primitive xe2x80x9cdevices,xe2x80x9d which are characterized by various compact or primitive model-level parameters. For example, a device may be a transistor or portions thereof, from which larger or more complex structures like op amps are composed. A chip thus comprises a circuit or system which comprises a network of circuit-level elements, which are themselves formed from the compact devices.
The technique of worst-case files is often used in order to model the interdie variations in a manner that is useful for circuit designers. Worst-case files represent a number of cases that include the nominal case and also various extreme cases, each of which consists of device model parameters corresponding to a particular xe2x80x9cprocessing corner.xe2x80x9d Collectively, the worst-case files represent the nominal and various extremes of device behavior corresponding to the variations of a particular manufacturing process. The use of worst-case files is described in C. Michael and M. Ismail, Statistical Modeling for Computer-Aided Design of MOS VLSI Circuits, Kluwer Academic Publishers, Boston/Dordrecht/London, 1992.
Obviously, a higher parametric yield is desirable so that more working chips are produced in each given lot. Worst-case files are thus used by chip designers to try to achieve high parametric yield. The widths and lengths of the various primitive devices may be adjusted, by repeated simulations and/or experimentation, to achieve a high percentage of chips that are expected that satisfy the worst-case limits. Designs that are predicted to work satisfactorily when simulated with worst-case files can be expected to have high parametric yields. Thus, by adjusting the sizes (widths and lengths) of the various primitive devices and verifying via simulation with the worst-case files that the performance is satisfactory, a high parametric yield can be expected from the resulting design.
Thus, a given circuit is designed and laid out by using worst-case files. The performance of the circuit design may then be simulated for the nominal case, to ensure it satisfies the key performance metrics. The performance for other cases of interdie variation may also be checked to see if all or most of them satisfy the desired key performance metrics. If all or most (to a certain specified percentage) of these cases also perform satisfactorily, then a high parametric yield can be expected since the worst-case interdie variations (variations in important or selected circuit performance metrics) caused by manufacturing variations will still allow the circuit to perform satisfactorily. These and other aspects of using worst-case files is discussed in the Michael and Ismail text and in D. Foty, MOSFET Modeling with Spice, Prentice Hall, Upper Saddle River, N.J., 1997.
Such an approach may be feasible for a circuit whose specifications are not very aggressive and whose performance is not very sensitive to interdie variations. Such circuits may be xe2x80x9cover-designedxe2x80x9d for the nominal case so that circuit performance is still satisfactory even when there is deviation from the nominal case for many or all of the circuit performance metrics. This can result in a circuit design expected to provide satisfactory performance even at all the extreme cases.
However, the worst-case file or xe2x80x9ccase-based simulationxe2x80x9d approach is not always feasible or optimal. For example, in some designs a number of complex, competing performance constraints may be specified. These constraints may be satisfied in the nominal case, but different performance criteria would violate their specifications to different degrees in the extreme cases. In such a situation, the case-based simulation approach does not provide the designer with any quantitative feedback on the robustness of the design. In this case, the designer may be forced to over-design or, if this is not feasible, the parametric yield will be unpredictable and possibly too low for economic viability.
The use of statistical device models (also sometimes known as statistical process models) can help to alleviate this problem. Statistical models of semiconductor devices are used to quantitatively assess the key circuit performance metrics which are expected to result from a mass manufacture in a given production process or foundry. In particular, a statistical device model allows one to predict the correlated variations of the relevant performance metrics of the population of chips to be manufactured via a given process. Thus, with a suitable statistical model it is possible to determine, to some degree of accuracy, the standard deviations and correlations of the various performance metrics of the product or system to be manufactured. The statistical device models can also allow one to more accurately determine the percentage of sample circuits that satisfy all the performance specifications, i.e., to predict the expected parametric yield. Thus, for a given schematic layout and IC design, the statistical model can be used to quantitatively assess the manufacturability of the IC design with respect to a target foundry.
Conventional approaches to statistical device modeling include Michael and Ismail; and P. Chatterjee, P. Yang, D. Hocevar and P. Cox, xe2x80x9cStatistical Analysis in VLSI Process/Circuit Design,xe2x80x9d in Statistical Approach to VLSI, ed. S. W. Director and W. Maly, pp. 255-292, North-Holland, 1994. Such approaches typically assume the availability of I-V (current-voltage) measurements on a large number of units. Device model parameters are then extracted for each measured chip using standard parameter extraction techniques. From this database, the correlated distributions of the model parameters are determined and form the basis of further statistical analysis.
These conventional approaches to statistical device modeling, however, may not always be practical or optimal. For instance, in a production environment, statistical device models that capture both nominal and extreme behavior are needed early in the life cycle of a production process. Worst-case files can be generated early, since they are, to a large extent, based on process specifications. However, with conventional techniques it is difficult to produce a good statistical device model early in the life cycle of the process.
So-called xe2x80x9cTechnology CADxe2x80x9d tools are sometimes used to derive a statistical device model that will mimic the anticipated spread of a device manufacturing process. However, such an approach requires a mature process so that a new, complete set of I-V measurements can be made on a large number of chips, and/or the use of such tools to develop a statistical model is very complicated and resource- and time-intensive.
A number of techniques for statistical modeling have been proposed to develop statistical models while attempting to avoid these problems, such as those described in S. Nassif, xe2x80x9cStatistical Worst-Case Analysis for Integrated Circuits,xe2x80x9d in Statistical Approach to VLSI, pp. 233-254; J. A. Power, B. Donnellan, A. Mathewson and W. A. Lane, xe2x80x9cRelating Statistical MOSFET Model Parameter Variabilities to IC Manufacturing Process Fluctuations Enabling Realist Worst Case Design,xe2x80x9d IEEE Trans. Semiconductor Manufacturing, v. 7, no. 3, pp. 306-318, August 1994; A. Bolt, M. Rocchi and Jan Engel, xe2x80x9cRealistic Statistical Worst-Case Simulation of VLSI Circuits,xe2x80x9d IEEE Trans. Semiconductor Manufacturing, v. 4, no. 3, pp. 193-198, August 1991. The primary thrust of such approaches relies on developing realistic worst-case files from a statistical analysis of certain data collected from a mature process. Such an approach typically requires, however, a mature process in order to collect the statistical data needed to derive the worst case files. This approach may not be helpful, therefore, early in the life cycle of the production process. Further, as described above, use of worst-case files for case-based simulation is not always desirable, no matter how the worst-case files are derived. There is, accordingly, a need for improved techniques for deriving statistical device models.
A set of worst-case device model files is provided for a production process used to mass-produce integrated circuits having a plurality of primitive device model types. A statistical device model for the production process is derived directly from the worst-case files.